The invention relates to metal forming, and more particularly to a method of rolling section billets.
The invention may be used in both existing and new reversible and continuous billet rolling mills and also in the construction of combined plants including a continuous metal casting machine and a rolling mill, as well as in the manufacture of section rolled products for multistrand rolling, e.g. in a roughing group of wire mills.
The invention enables an improvement of output of existing rolling mills and construction of new highly efficient combined plants including a continuous metal casting machine and a rolling mill.
In many industrially developed countries, the demand for various section rolled products which are manufactured from section billets has been increasing due to the enormous growth of metal consuming industries.
The need to improve the output of reducing and billet rolling mills has brought about new methods for the manufacture of billets.
Nowadays, the method of rolling section billets is in accordance with a single ingot-blooming-continuous billet rolling mill which is generally the one most widely used. Substantially all known methods of improving the output of such devices have reached their limit.
At present, the manufacture of section billets in continuous metal casting machines is rapidly developing.
The manufacture of billets directly by continuous casting method is only possible using continuous metal casting machines having a large number of strands, which requires large capital investments.
More efficient is a method of continuous casting of large-section billets with subsequent rolling thereof in continuous billet rolling mills. This method, however, also requires large investments.
The use of a less known method of producing section billets by reducing continuously cast slabs in reducing plants is also limited due to a number of important disadvantages.
Most promising and highly efficient method of the manufacture of section billets consists in rolling and splitting a rolled or continuously cast wide billet in multipass grooves defined by a plurality of transversely juxtaposed grooves.
Known in the art is a method of making twin billets, in which a continuously cast rectangular-section slab is rolled between a number of twin grooves arranged in series, the cross-sectional shape of the groove gradually approximating a square of rhombus. The resultant billets are then cut apart by means of gas cutters.
While having the advantages of twin rolling of billets, this method is deficient in prevailing reduction of billets in the bridge zone. Longitudinal stresses appearing in less reduced central portions of the billets result in surface flows and internal flaws.
Also known in the art is a method for improving quality of rolled products in which an ingot of square or almost square section is cast and then rolled into a plurality of billets transversely interconnected by bridges in such a manner that phase separation zone remains limited to the middle portion or middle portions of the cross-section, and then the section product is separated at the bridges into individual products free of phase separation zones and containing phase separation zones.
It should be noted that this method is deficient in that the quality of metal is impaired during rolling due to considerable tensile stresses in the central portions of the billets in all passes, which results in the uncovering of surface flaws.
Another prior art method of continuously making section billets comprises the step of subjecting a continuous slab leaving a mould to a successive reduction between a series of calibrated rolls until complete solidification. As a result of deformation of wider sides, the slab takes a shape of a plurality of interconnected square billets. After secondary cooling with sprayed water, the billets are cut lengthwise and transversely to obtain measured lengths.
The disadvantage of this method resides in a trend to the formation of surface flaws, which is more pronounced due to the presence of a liquid core.
A further prior art method of multistrand rolling of metal section products comprises a multistrand rolling of two or more section products, with subsequent splicing by cutting-off a thin metal bridge in either the hot or, preferably, cold state.
This highly efficient method of multistrand rolling is, however, deficient in prevailing reduction of billets in the bridge zone in all grooves, e.g. in rolling square billets. This results in the appearance of considerable longitudinal tensile stresses in the less reduced central portions of the billets and in surface flaws in these portions. This trend is most pronounced in rolling borely deformable grades of steel by the multistrand method.
It is an object of the invention to eliminate the above-mentioned disadvantages.
Another object of the invention is to provide a method of multistrand rolling of section billets with guaranteed quality of metal continuity, which depends on the quality of the starting billet, due to a reduction of longitudinal tensile stresses.
The invention consists in the provision of a method of multistrand rolling of section billets with guaranteed quality of metal continuity which depends on the quality of the starting billet, due to a reduction of longitudinal tensile stresses.